Sintered refractory material based on silicon carbide with a silicon nitride binder

ABSTRACT

A sintered material based on silicon carbide (SiC) reactively sintered between 1,100° C. and 1,700° C. to form a silicon nitride binder (Si 3 N 4 ), intended in particular for fabricating an aluminum electrolysis cell, including 0.05% to 1.5% of boron, the Si 3 N 4 /SiC weight ratio being in the range 0.05 to 0.45. Application, in particular, to an electrolysis cell.

This patent application is continuation in part of the patentapplication filed in USA on Nov. 25, 2005 under Ser. No. 11/791,653(PCT/FR2005/002936) in the name of SAINT GOBAIN CENTRE DE RECHERCHES ETD'ETUDES EUROPEEN.

The invention relates to novel sintered refractory blocks, especiallyfor the construction of aluminum electrolysis cells, to a method ofmanufacturing them, and to a cell comprising said blocks.

As can be seen in FIG. 1, aluminum metal 2 may be produced on anindustrial scale by electrolyzing alumina in solution in a bath 10 basedon molten cryolite. The electrolyte bath 10 is conventionally containedin an electrolysis cell 12. The cell 12 comprises a side wall 14 and abottom 16. The bottom 16 is composed of refractory bottom blocks 17 andcathode blocks 24 and insulating blocks in the lower portion. The sidewall 14 is formed from refractory side blocks 18 surrounded by a metalcasing 20.

The dimensions of a refractory side block 18 can vary. They areconventionally more than 30×100×100 mm [millimeter] and may attain120×300×300 mm.

The composition of the blocks 18 may be based on carbon (graphite and/oranthracite). Typically, the mortar for the blocks 18 is a refractorycement 21 disposed between them and against the metal envelope 20. Thecell 12 comprises at least one anode 22 and at least one cathode 24. Theanodes 22 and cathodes 24 are disposed so as to be in contact with themolten metal bath, the cathode 24 conventionally being disposed close tothe bottom 16.

When the electrodes 22 and 24 are placed under voltage, an electrolysisreaction occurs in the bath 10, resulting in the formation of a bath ofaluminum in the cell, which bath is deposited on the cathode.

Passing high electric current through the bath 10 also causes heat to bereleased under the Joule effect. Evacuating that heat through the wall14 of the cell 12 causes a layer 26 of solidified cryolite to bedeposited on the inner surface 27 of the blocks 18. That layer is termeda “self-lining” layer.

The blocks 18 must protect the metal envelope 20 and allow sufficientheat to be evacuated to ensure temperature stabilization of the moltenbath 10. In particular, it is vital to avoid reaching temperaturesbeyond which the self-lining layer 26 of solidified cryolite liquefiesagain and could contribute to very rapid corrosion of the sides of thecell. Further, the blocks 18 are often exposed to corrosive environments(very hot liquid metal, molten cryolite in the lower portion, corrosivegas in the upper portion), and they are subjected to high temperaturesand large thermal and mechanical stresses.

To meet those challenges, blocks are known that are based on siliconcarbide granulates which have generally satisfactory resistance toattack. Conventionally, silicon carbide granulates are sintered at atemperature in the range 1600° C. to about 2000° C. Sintering finegrained silicon carbide granulates at very high temperatures (2150° C.)is also known, enabling boron and carbon to be added. However, siliconcarbide is very difficult to sinter and/or its cost is prohibitive.Further, the format of the sintered silicon carbide blocks is limited,especially due to a great deal of shrinkage on firing.

Blocks based on dense sintered silicon carbide granulates are alsoknown, with less than 1% of B₄C and C, for example Hexolloy SiC®.However, they are currently extremely expensive.

Finally, blocks based on silicon carbide (SiC) are known, bound by amatrix of silicon nitride (Si₃N₄). The materials for such blocks weredeveloped at the end of the 1970s and are described, for example, inU.S. Pat. No. 2,752,258. They improve the compromise between oxidationresistance, mechanical strength (erosion), and thermal conductivitycompared with carbon blocks. The improvement in abrasion resistance isparticularly advantageous at the bottom of the cell where the bath,which moves under the effect of magnetic fields, may cause a great dealof abrasion.

Said blocks are obtained by reactive sintering of a mixture of siliconcarbide and silicon, with nitrogen deriving from firing in a nitrogenatmosphere.

By “reactively sintered product” is meant a ceramic product having anitrogenous matrix, whether crystallized or not, obtained during thethermal treatment (known also as sintering) from precursors introducedwithin the starting charge. These precursors are preferably in the formof powders having a median diameter of less than 200 microns, andpreferably contain silicon. By definition, a matrix ensures anessentially continuous structure between the silicon carbide grains.

To gain useful volume and facilitate heat evacuation, research has beenconcentrated on reducing the thickness of such blocks. However, thethickness cannot be reduced without affecting the service life of thecells. Thus, it must be accompanied by an improvement in the oxidationresistance and resistance to attack by the cryolite bath. That need isgreater if the stresses on the refractory blocks are higher. Inparticular, electrolysis cells are now used with a current of more than200,000 amps and from which, as a result, a great deal of heat must beevacuated, large quantities of oxidizing gas are generated, and theself-lining layer may become unstable.

Thus, there is a need for a novel refractory block based on siliconcarbide (SiC) with a nitride binder (Si₃N₄) that can effectively anddurably resist the thermal and/or chemical stresses that may be producedin an aluminum electrolysis cell, in particular in the side wallthereof.

The invention aims to answer this need.

According to the invention, this aim is achieved by means of a sinteredrefractory block based on silicon carbide (SiC) with a silicon nitride(Si₃N₄) binder, in particular intended for fabricating an aluminumelectrolysis cell, which block is remarkable in that it includes, as apercentage by weight, a total calcium and boron content in the range0.05% to 1.5%, preferably 1.2%. Preferably, it includes at least 0.05%,preferably at least 0.3%, and more preferably at least 0.5% of boron,and/or in the range 0.05% to 1.2% of calcium.

Surprisingly, the inventors have discovered that the presence of boronand/or calcium provides a substantial improvement in properties asregards aluminum electrolysis cell applications, in particularresistance to oxidation and to attack by the cryolite bath, anddimensional stability under oxidation conditions.

The refractory block of the invention also exhibits one or more of thefollowing preferred characteristics:

-   -   the refractory block includes less than 3% of boron, as a        percentage by weight;    -   silicon nitride (Si₃N₄) in the beta form represents, as a        percentage by weight, at least 40%, preferably at least 60%, and        more preferably at least 80%, of all of the silicon nitride        (Si₃N₄) in the beta form and in the alpha form;    -   the Si₂ON₂ content, as a percentage by weight, is less than 5%,        preferably less than 2%;    -   the porosity of the sintered block is preferably 10% or more;        and    -   the boron is not in the TiB₂ form, as that form of titanium is        not stable in contact with molten cryolite, in an oxidizing        atmosphere. Further, TiB₂ is also unstable towards aluminum.

Preferably again, the Si₃N₄/SiC weight ratio is in the range 5% to 45%,preferably in the range 10% to 20%, i.e. in the range 0.05 to 0.45,preferably in the range 0.1 to 0.2.

Preferably, the Si₃N₄/SiC ratio is less than 0.3 and/or more than 0.05.Further, the Si₃N₄ content is preferably 11% or more, as a percentage byweight.

The invention also provides an electrolysis cell including a side wallcomprising a plurality of refractory blocks, at least one of said blocksbeing in accordance with the invention. Preferably, all of the blocksforming the side wall of the cell of the invention are in accordancewith the invention.

In particular for this application, a block in accordance with theinvention does not, preferably, contain any alumina.

In the other applications of the invention, and in particular for kilnfurniture and particularly supports to sinter porcelain pieces, a block,and more generally a refractory product in accordance with theinvention, may contain at least 0.8%, at least 1% of alumina. It mayalso contain more than 0.1%, or more than 0.2% of Fe₂O₃.

Preferably, a powder of calcined alumina is added in the startingcharge. Preferably, the mean size of added alumina powder ranges between0.4 and 10 microns

Finally, the invention provides a method of fabricating a refractoryblock in accordance with the invention, comprising the following stepsin succession:

a) preparing a charge comprising a particulate mixture comprising asilicon carbide granulate and at least one boron and/or calciumcompound, a binder optionally being added to said particulate mixture;

b) forming said charge in a mold;

c) compacting said charge in the mold to form a preform;

d) unmolding said preform;

e) drying said preform, preferably in air or a moisture-controlledatmosphere; and

f) firing said preform in a reducing atmosphere of nitrogen at atemperature in the range 1100° C. to 1700° C.

The inventors have discovered that adding boron and/or calcium to theformulations improves the properties of sintered silicon carbide (SiC)based refractory blocks with a silicon nitride binder (Si₃N₄) which areobtained. In particular, the resistance to corrosion byfluorine-containing products and molten cryolite is improved.

The method of the invention also has one or more of the followingpreferred characteristics:

-   -   said boron and/or calcium compound contains boron;    -   said boron and/or calcium compound is added in a predetermined        quantity so that the refractory block obtained at the end of        step f) is in accordance with the invention, in particular so        that it comprises, as a percentage by weight, at least 0.05%,        preferably at least 0.3%, more preferably at least 0.5% of        boron, and/or less than 3% of boron;    -   said boron and/or calcium compound is free of oxygen, i.e. added        in a “non-oxide form”;    -   said boron compound is selected from the group formed by oxides,        carbides, nitrides, fluorides and metal alloys containing boron,        in particular B₄C, CaB₆, H₃BO₃, and BN, preferably from the        group formed by B₄C and CaB₆. More preferably, said boron        compound is CaB₆;    -   said calcium compound is selected from the group formed by        oxides, carbides, nitrides, fluorides and metal alloys        containing calcium, preferably selected from CaB₆, CaSi, CaSiO₃,        and CaCO₃;    -   said calcium compound is added in a predetermined quantity such        that the calcium content of the refractory block obtained at the        end of step f) is in the range 0.05% to 1.2%, as a percentage by        weight.

Other characteristics and advantages of the present invention becomeapparent from the following description, made with reference to theaccompanying drawings in which:

FIG. 1 is a diagrammatic representation of an electrolysis cell in crosssection along a substantially medial plane;

FIG. 2 represents in the form of graphs, the percentage variation in theincrease in volume due to oxidation as a function of time for differentblocks tested in accordance with American Standard ASTM C863 at 900° C.;

FIG. 3 represents schematically the device used for the test 8.

Unless otherwise indicated, all of the percentages in the presentdescription are percentages by weight.

When a granulate is said to be “based on” a constituent, this means thatsaid granulate comprises more than 50% by weight of that constituent.

Known methods of fabricating refractory blocks may be employed tofabricate a block in accordance with the invention, provided that atleast one oxygen-free boron compound is added to the starting charge.

Preferably, the method employed comprises the following steps:

a) preparing a charge comprising a particulate mixture comprising asilicon carbide granulate and at least one boron and/or calciumcompound, a binder being added to said particulate mixture;

b) forming said charge in a mold;

c) compacting said charge in the mold to form a preform;

d) unmolding said preform;

e) drying said preform, preferably in air or a moisture-controlledatmosphere, using conventional preform fabrication procedures; and

f) firing said preform in a reducing atmosphere of nitrogen at atemperature of 1100° C. to 1700° C., and drying.

In step a), the particulate mixture preferably comprises, as apercentage by weight, 30% to 90% of refractory grains wherein at least90% have a size in the range 50 μm [micrometer] to 5 mm [millimeter],and 10% to 60% of at least one refractory powder wherein at least 90% ofthe particles have a diameter of less than 200 μm. Advantageously, saidgranulometric distribution can endow the fabricated block with optimumcohesion.

The boron may be supplied in a particulate form or in any other formprovided that the maximum moisture content of the mixture remains below7%, preferably below 5%.

The function of the binder is to form with the particulate mixture amass that is sufficiently rigid to preserve its shape until step e). Thechoice of binder depends on the desired shape. Because of the binder,the mass may advantageously take the form of a layer of varyingthickness, which can follow the wall of the mold, to form blocks.

Any known binder or mixture of known binders may be used. The bindersare preferably “temporary”, i.e. they are completely or partiallyeliminated during the block drying and firing steps. More preferably, atleast one of the temporary binders is a solution of modified starchderivatives, an aqueous solution of dextrin or of lignone derivatives, asolution of a processing agent such as polyvinyl alcohol, a phenol resinor another epoxy type resin, a furfuryl alcohol, or a mixture thereof.More preferably, the quantity of temporary binder is in the range 0.5%to 7% by weight relative to the particulate mixture of the charge.

Pressing additives as are conventionally used in fabricating sinteredblocks may be added to the particulate mixture and the binder. Saidadditives comprise plasticizers, for example modified starches orpolyethylene glycols and lubricants, for example soluble oils orstearate derivatives. The quantities of such additives are thoseconventionally used when fabricating sintered silicon carbide (SiC)based refractory blocks with a silicon nitride binder (Si₃N₄).

Mixing of the charge is continued until a substantially homogeneous massis obtained.

In step b), the charge is shaped and placed in a mold.

In the next compaction or “pressing” step c), the contents of the moldare compressed by applying a force to the upper surface of the chargewhich can transform it into a preform that is capable of being sintered.A specific pressure of 300 kg/cm² [kilogram/square centimeter] to 600kg/cm² is appropriate. Pressing is preferably carried out uniaxially orisostatically, for example using a hydraulic press. It mayadvantageously be preceded by a manual or pneumatic and/or vibrationalramming operation.

Next, the preform is unmolded (step d)), then dried (step e)). Dryingcan be carried out at a moderately high temperature. Preferably, it iscarried out at a temperature in the range 110° C. to 200° C. Itconventionally lasts between 10 hours and one week, depending on theformat of the preform, until the residual moisture content of thepreform is less than 0.5%.

The dried preform is then fired (step f)). The firing period, betweenabout 3 and 15 days cold to cold, depends on the materials and also onthe size and shape of the block. In accordance with the invention,firing is carried out in nitrogen in order to form the nitride byreactive sintering, which nitride acts as the ceramic binder for thegrains. The firing cycle is preferably carried out at a temperature inthe range 1100° C. to 1700° C. During firing, nitrogen reacts withcertain of the constituents of the particulate mixture of the charge toform a matrix of silicon nitride which can bind the grains of saidcharge, in particular grains of silicon carbide. A monolithic block isproduced.

In the various tests below, provided by way of non-limitingillustration, the particle size of the powders used as additives (B₄C,CaB₆, and CaSiO₃) is less than 45 μm. Their respective contents in thestarting composition are indicated in Table 1.

Metallic silicon is also added in the proportion indicated in Table 1.

“Black” or “refractory” silicon carbide with different granulometricfractions, sold by Saint-Gobain Ceramic Materials, was also used. Thismaterial is essentially constituted by alpha SiC and has a mean chemicalanalysis, by weight, of 98.5% of SiC.

Table 1 also shows the results of various tests carried out tocharacterize the products of the invention compared with the referenceproduct (Refrax® type product). All of the measurements were carried outon the core of the specimens.

-   -   The nitrogen (N) contents in the products were measured using        LECO (LECO TC 436 DR; LECO CS 300) type analyzers. The values        given are percentages by weight.    -   The boron (B) and calcium (Ca) contents in the products were        measured by X-ray fluorescence spectrometry. The values given        are percentages by weight.    -   The oxidation tests were carried out in accordance with ASTM        C863. To reproduce the oxidation conditions experienced by the        blocks of an aluminum electrolysis cell, the specimens        (typically with a size of 25×25×120 mm) underwent a test of at        least 100 hours at 900° C. in an atmosphere saturated with        steam. Oxidation generates an increase in mass (“Om” value,        given as a percentage) and/or in volume (“Ov”, given as a        percentage), which results from the transformation of the        silicon nitride and carbide into silica. The increases in mass        and volume were thus indicators of the degree of oxidation. Two        materials were considered to be different when their oxidation        indicators differed by at least 1% (mean over 3 test specimens).    -   The variation in open porosity due to plugging by the oxidation        products of the reaction was also a measure allowing the degree        of oxidation to be determined. The open porosity was measured in        accordance with International Standard ISO5017 (“PO-Ox” value,        given as a percentage).    -   The corrosion resistance test allowed the behavior of 25 mm×25        mm cross section specimens which had already undergone the        oxidation test to be determined. These specimens were kept at        1030° C. for 22 hours in a bath of molten cryolite. Their        corroded length was then measured, i.e. the reduction in their        length resulting from corrosion. The value “Ic” provides, as a        percentage, the ratio between the corroded length of the test        specimen and the corroded length of a reference specimen. The        lower the Ic, the better the corrosion resistance.    -   The crystalline phases present in the refractory products were        determined by X ray diffraction. Principally, it was found that        silicon nitride Si₃N₄ as well as an oxynitride phase, Si₂ON₂,        were present. The amounts of these phases, as a percentage by        weight, are indicated in Table 1. The complement was SiC.

The apparent specific gravity of the products of the invention was inthe range 2.4 to 2.7. That of the reference product was 2.6.

The silicon nitride could be in the alpha or beta form. The alpha phasewas present in the form of a tangle of silicon nitride fibrils, whilethe beta phase was in the form of grains with a variable shape.

Analyses carried out over several years by the Applicant have shown thatsilicon nitride in the beta form is less sensitive to rapid combustionbecause of its lower specific surface area, than is silicon nitride inthe alpha form. During rapid combustion, silicon nitride is oxidized andproduces silica which is “consumed” by the molten cryolite. Thosereactions thus result in an increase in the porosity and connectivity ofthe pores, facilitating the penetration of corrosive materials. Thus, itis advantageous to encourage the formation of the beta form to improvethe resistance to attack by molten cryolite.

However, it is known that enrichment of the beta silicon nitride phaseis generally accompanied by an enrichment in the oxynitride phaseSi₂ON₂.

However, silicon oxynitride, like residual silicon and Sialon withalumina impurities, conventionally generated during a nitriding process,are unwanted phases which have a lower resistance to cryolite comparedwith that of silicon nitride, regardless of the form of the siliconnitride. Thus, it is advantageous to limit the quantities thereof.

The inventors have discovered that adding boron and/or calcium,preferably in an oxygen-free form, to the starting charge advantageouslystimulates the transformation into beta silicon nitride during theprocess for nitriding silicon carbides with a nitride binder, withoutcausing deleterious enrichment of the Si₂ON₂ oxynitride phase. Table 1below illustrates this discovery.

In accordance with the invention, a boron compound is thus added,preferably in a non-oxide form. Advantageously, this addition results inalmost complete transformation into beta silicon nitride without majorenrichment of the oxynitride phase Si₂ON₂.

TABLE 1 Composition Analysis N° Si B₄C CaB₆ H₃BO₃ CaCO₃ PO N B Ca Si₃N₄a Si₃N₄ b b/a Si₂ON₂ Om Ov PO—Ox Ic R 13.5 0 0 0 0 17.2 5.9 0.0 0.1 12  5 29 3 1.6 1.0 14.0 100  1 13.5 0.2 0.0 0.0 0.0 16.2 6.4 0.1 0.1 10   947 3 2.4 0.0 9.4 45 2 11.8 0.5 0.0 0.0 0.0 17.1 7.4 0.5 0.1 8  9 53 32.7 0.1 9.5 ND 3 13.5 0.5 0.0 0.0 0.0 15.5 6.1 0.4 0.2 ND ND ND ND 1.30.0 8.6 65 4 10.0 0.8 0.0 0.0 0.0 16.8 5.5 0.6 0.1 7  7 50 ND 2.5 0.06.7 61 5 13.5 0.8 0.0 0.0 0.9 16.3 7.1 0.5 0.4 1 14 93 4 1.5 0.0 10.0 415 13.5 0.8 0.0 0.0 0.0 16.1 7.0 0.5 0.0 2 14 88 4 1.9 0.1 9.4 41 7 13.50.8 0.0 0.0 0.0 16.1 7.1 0.6 0.1 7 10 59 2 2.4 0.1 6.2 45 8 14.3 0.8 0.00.0 0.0 16.1 7.0 0.5 0.0 1 14 93 4 2.0 0.1 9.5 41 9 13.5 1.0 0.0 0.0 0.015.2 6.1 0.6 0.2 ND ND ND ND 1.5 0.1 7.6 72 10 13.5 1.6 0.0 0.0 0.0 16.16.9 ND 0.2 2 17 89 0 2.2 0.2 7.1 70 11 14.3 1.6 0.0 0.0 0.0 17.0 7.2 1.00.1 1 14 93 4 2.3 0.0 7.3 70 12 13.5 3.0 0.0 0.0 0.0 13.6 4.5 1.9 0.2 NDND ND ND 1.4 0.0 8.3 60 13 14.3 5.0 0.0 0.0 0.0 14.1 8.1 2.9 0.1 1 14 934 1.7 0.3 5.1 73 14 13.5 0.0 0.0 3.5 0.0 20.0 7.1 0.4 0.1 2  9 82 7 1.00.0 13.5 86 15 13.5 0.0 0.0 3.5 0.9 20.0 7.8 0.4 0.4 2 13 87 7 1.4 0.316.4 88 16 13.5 0.0 0.0 0.0 2.0 19.1 6.7 ND 1.0 9  9 100  0 1.5 0.9 ND75 17 13.5 0.0 0.2 0.0 0.0 16.6 7.3 0.1 0.2 8 10 56 2 1.9 0.2 10.3 45 1813.5 0.0 0.5 0.0 0.0 14.3 5.8 0.3 0.5 ND ND ND ND 1.0 0.0 8.3 ND 19 13.50.0 1.0 0.0 0.0 16.4 7.7 0.3 0.4 1 18 95 0 1.6 0.3 11.3 35 20 13.5 0.01.0 0.0 0.0 15.6 7.1 0.6 0.5 3 15 83 3 1.7 0.0 7.7 47 21 13.5 0.0 3.00.0 0.0 14.3 6.0 1.1 1.4 ND ND ND ND 2.0 0.0 7.5 ND

Table 1 indicates that adding boron and/or calcium can improve thecorrosion resistance of refractories formed from silicon carbide with aSi₃N₄ matrix binder.

Table 1 indicates that adding boron and/or calcium can advantageouslyenhance the proportion of beta phase Si₃N₄. However, only adding boronand/or calcium in a non oxide form can limit the amount of siliconoxynitride Si₂ON₂ to values close to or lower than that of the referenceproduct, as can be seen in Examples 14 and 15.

Table 1 indicates that the open porosity is improved when the amount ofboron in the final product is non zero: only examples 14, 15 and 16 havean open porosity that is greater than that of the reference product. Forthis reason, products containing 0.05% to 3% of boron are preferred.

Further, Table 1 shows that the presence of boron compounds in thestarting charge advantageously catalyzes the nitriding reaction (theamount of nitrogen in the products of the invention is enhanced).

Without wishing to be bound by a theory, the inventors have a partialexplanation for the improvement in corrosion resistance, namelystabilization of oxidation degradation. As indicated in Table 1 andshown in FIG. 2, the oxidation resistance of the products of theinvention is improved.

Table 1 shows that the variation in volume due to oxidation is verylimited in the products of the invention. Further, the increase in massafter oxidation is limited when the calcium that is added is combinedwith boron. Thus, adding CaB₆ is advantageous, in particular in amountsbetween 0.5% and 2%.

FIG. 2 shows the change in the resistance to oxidation when the test isextended to 500 hours. The improvement over the reference is confirmedand accentuated.

Table 1 shows that adding boron and/or calcium has an influence oncorrosion resistance even for small amounts. It also appears that aminimum amount of 0.8% by weight can produce a substantially maximumcorrosion resistance.

Preferably, the percentage by weight of CaB₆ in the starting charge ismore than 0.5%.

It appears that the effect of adding B₄C to the starting charge issubstantially the same at B₄C contents as low as 0.2%. A reinforcedeffect is obtained with a content of 0.6%.

Clearly, the present invention is not limited to the implementationsdescribed and shown by way of non limiting illustrative examples.

The inventors have also observed that adding boron in the non oxideform, and more specifically CaB₆ or B₄C, also contributes to increasingthe thermal diffusivity of the products of the invention, without aspecific effect linked to compactness. This is clearly very important inencouraging heat transfer.

Further, it has been found that none of the products of the inventioncontains Si₃N₄ in the acicular form, including on the surface.

The oxidation performance of the products of the invention indicatesthat applications other than in electrolysis cells may be envisaged.

But, the inventors have discovered that the material of the refractoryblock of the invention which has been described here above, called“sintered material of the invention” may be used, with various forms, inquite different applications than in electrolysis cells and, inparticular, in applications where it could not be predicted that thismaterial would be appropriate.

The addition of boron in a refractory product usually lowers the meltingtemperature of this product. The possibility of using of a product ofthe invention in applications where it may be submitted to temperaturesthat may reach 1 500° C., 1 600° C. or even 1 700° C. was thereforesurprising.

The inventors have also discovered that the resistance to oxidation ofthe products of the invention is much less variable from one product tothe other in comparison with corresponding products which would notcontain any boron.

The invention also concerns a refractory product, in the form of:

-   -   a “shaped product”, in particular in the form of a block, for        instance a brick or a wall block, a plate, a tile, and a tube,    -   a “non shaped product”, in particular a lining,        comprising, or made of, a sintered material of the invention,        said sintered material being manufactured on plant, for instance        from a castable or a pumpable composition.

A shaped product is a product which has been given a specific shape witha mold or through a die. On the contrary, an unshaped product is aproduct which has been applied on a support without being shaped, forinstance through a projection of a castable product. An unshaped productis sintered after being applied on the support.

The shaped product may be chosen in the group consisting of kilnfurniture or of other firing supports, in particular in the form of aplate, a post, a setter, a beam or a roller, said kiln furniture beingcovered by a protective coating or not; a block, a tile and a tube of aheat exchanger; a block, a tile and a tube of a heat recuperator; ablock, a tile and a tube of a heat regenerator; a block, a tile and atube of a muffle; a block, a tile and a tube of a domestic incinerationfurnace; a block, a tile and a tube of a furnace of a glass makingfactory, in particular in a location where the sintered material of theinvention is not in direct contact with the molten glass, a block, atile and a tube of a metallurgical furnace, in particular a block, atile and a tube of a holding furnace or a block of a smelting furnacefor non ferrous metals, like copper shaft furnaces; a block, a tile anda tube of a protection wall, in particular to protect a heat exchanger;a block of a skid rail of a metallurgical furnace, for example a furnacefor the heat treatment of iron or steel bars or of other metallurgicalproducts; a block and a tube of an immersion heater; a heater tube, athermocouple protective tube, and a tube for transporting molten metal.

The shaped product may have any shape, depending on the intendedapplication.

In particular, it may be massive.

The maximal thickness of a product is usually determined so that thelevel of residual silicon in the core of the product be less than 1% byweight, as measured for example by X Ray diffraction. It depends on thefiring technology. Preferably, the maximal thickness is less than 150mm.

The minimal thickness of a product is mainly depending on the formingtechnology, the mix formulation and the desired mechanical properties.

Surprisingly, a shaped product of the invention may be thin, forinstance substantially flat. Its thickness may be less than 3/10, lessthan 2/10, less than 1/10, less than 5/100, less than 2/100 or even lessthan 1/100 or less than 5/1000 of its length and/or of its width.

The invention in particular relates to a refractory product, forinstance in the form of a plate, the thickness of which is less than 10mm, less then 8 mm, or less than 5 mm. It may have a length and/or awidth of 5 cm or more, 10 cm or more, 20 cm or more, 50 cm or more oreven 100 cm or more. Its surface may be 1 m² or more.

Without being bound by a theory, the inventors consider that boron helpsto form a stable nitrogen glassy phase that limits the increase ofcrystallized silica (SiO₂) and therefore avoids a fall of the mechanicalresistance in service. A sintered material of the invention maytherefore be used in the form of a thin product.

The invention also concerns a device chosen in the group constituted ofa heat exchanger, an incineration furnace, in particular for domesticwaste, a heat recuperative furnace, a heat recuperator, a heatregenerator, a furnace for making heat insulation fibre glass, a firingsupport, and in particular kiln furniture, a glass making factory, ametallurgical furnace, in particular a holding furnace or a smeltingfurnace, a skid rail for a metallurgical furnace, an immersion heater,an assembled wall, an assembled lining, an assembled muffle and moregenerally an assembly of shaped products, a non shaped lining, i.e.which does not result of an assembly of shaped products, said devicecomprising, and possibly being constituted of, a sintered material ofthe invention.

Kiln Furniture

The inventors have discovered that the sintered material of theinvention has a very good mechanical resistance in a large range oftemperatures, especially from 800° C. up to 1,600° C., or even up to1,750° C. This was not expected because of the low smelting points ofcurrent oxide glassy phases containing boron.

They also discovered that the sintered material of the invention canvery well withstand cyclic variations of temperature. For instance itcan very well withstand more than 100 cycles between 20° C. and 1500° C.under air, each cycle lasting several hours, as it occurs in tunnelfurnaces for firing porcelain articles.

The invention therefore also concerns the use of a refractory productaccording to the invention (comprising, or made of, a sintered materialof the invention), in particular a shaped product of the invention, inan application where said product is submitted to cycling variations oftemperature, each cycle lasting less than 24 hours, in particular inapplications where in any cycle, the temperature varies of at least1000° C., of at least 1300° C. or even of at least 1500° C. and/or inwhich any cycle lasts at least 1 hour, at least 5 hours, or at least 10hours.

They finally discovered that said sintered material presents a very highdimensional stability. In fact, kiln furniture made of the sinteredmaterial of the invention has a long service life.

Besides, as described, for example, in U.S. Pat. No. 6,143,239 or EP 603851, incorporated by reference, it is known that kiln furniture may bemade of a support covered by a protective coating (in particular a solor a glaze), deposited on the fired support. The coating may comprise orbe made of metal hydrate, e.g. alumina hydrate, alumina,aluminosilicate, in particular mullite and its precursors, borosilicate,zircona, and mixture of these constituents.

The dilatation coefficients of the support and of the coating aredetermined to be as close as possible, but the difference of thesedilatation coefficients usually increases with time. In addition, thesupport may react with the coating and swell, which may lead to peelingof the coating. The inventors have observed that supports made of asintered material of the invention and covered with a coating such asthose described in U.S. Pat. No. 6,143,239 or EP 603 851 present a verylong service life, without peeling. Without being bound by this theory,they consider that this advantageous property is due to the fact thatthe sintered material of the invention does not react, or very little,with the coating.

The invention therefore also concerns the use of a refractory shapedproduct according to the invention (comprising, or made of, a sinteredmaterial of the invention), said refractory shaped product being atleast partly covered with a coating, to avoid peeling of said coating,in particular when said product is used as a support to sinter porcelainpieces.

Because of its noteworthy chemical inertness, the sintered material ofthe invention may be used, very efficiently, without any coating.Advantageously, the manufacturing and maintenance costs of kilnfurniture made of a sintered material according to the invention,without being coated, would be reduced.

The invention therefore also concerns a firing method in which an itemto be fired is supported by a firing support during firing, said firingsupport comprising a sintered material of the invention and being freeof any coating, at least on the surface in contact with said item.

Besides, in some applications, the firing support is classically firedunder air atmosphere, typically between 800° C. and 1,600° C. in orderto passivate the support. A firing under an oxidizing atmosphere, inparticular air, in order to passivate the support may be performedduring the cooling, and/or in the same furnace, after firing of saidsupport in a reducing atmosphere of nitrogen at a temperature of 1100°C. to 1700° C.

The inventors have discovered that the presence of boron in a productaccording to the invention improves the efficiency of the passivation.Therefore, the invention also concerns a method comprising steps a) toe), and a passivation step between 800° C. and 1,600° C.

The inventors have discovered that this operation may be omitted with aproduct of the invention.

Therefore, the invention also concerns a firing method in which an itemto be fired is supported by a firing support during firing, said firingsupport comprising a refractory product according to the invention(comprising, or made of, a sintered material of the invention) and beingfree of any coating, at least on the surface in contact with said item,and wherein said firing support has not been passivated.

Heat Recuperative Furnaces and Heat Recuperators

Heat recuperative furnaces are used for making glass, as described inthe patent application U.S. Pat. No. 4,497,628, incorporated byreference. The blocks of these furnaces, and in particular the bottomblocks, are exposed to corrosive alkali vapours at about 1000-1100° C.and to strong mechanical erosion due to high air velocities and to thepresence of particles or dust. Usually, the blocks used in heatrecuperative furnaces for making glass are therefore oxide products.

However, surprisingly, the inventors also discovered that the sinteredmaterial of the invention has an excellent thermal stability and a goodcorrosion resistance in the severe conditions which occur in thisapplication.

The invention also concerns the use of a sintered product of theinvention (comprising, or made of, a sintered material of the invention)in a bottom part of an heat regenerator of a heat recuperative furnacefor making glass.

Muffle

The muffles covering the forming blocks described, for example, in thepatent application US 2005/130830, incorporated by reference, aresubjected to a strong oxidizing environment during the manufacture ofglass. The inventors have discovered that the sintered material of theinvention is very stable in these severe conditions. The inventiontherefore also concerns a muffle, and more generally, any part of aglass making factory which is not in contact with molten glass,comprising, or made of, a sintered material of the invention.

Refractory Tubes

The inventors have found that the performances of refractory tubes likeheater tubes or thermocouple protective tubes or tubes for transportingmolten metal, as described for example in U.S. Pat. No. 5,135,893,incorporated by reference, may be improved by a sintered material of theinvention.

Incineration Furnace and/or Heat Exchangers

Refractory linings for incineration furnaces are for example describedin EP 0 107 520, WO 00/26597 or EP 0 635 678, incorporated by reference.

The inventors have discovered that a sintered material of the inventionis also very well suited to make the refractory lining of anincineration furnace, in particular for the incineration of domesticwaste. It may be shaped into refractory tiles or walls or tubes orbricks or the like. The sintered material of the invention indeedexhibits few cracks and a good corrosion resistance at temperatures upto 1,300° C. in the environment of such incineration furnaces.

The invention therefore also concerns the use of a refractory product ofthe invention (comprising, or made of, a sintered material of theinvention), in a refractory lining of an incineration furnace in whichsaid product has not been submitted to an oxidative post-treatment at atemperature between 1000° C. and 1700° C.

The inventors have also discovered that the thermal conductivity is verylittle reduced by the corrosion by alkalis vapours. This is veryadvantageous in applications where the material of the invention isintended to transfer the heat by conduction. The inventors have noticedthat an amount of boron of 1.5%, in percentage by weight of the sinteredmaterial according to the invention, enables a good maintenance of thethermal conductivity.

The invention therefore also concerns the use of a refractory product ofthe invention (comprising, or made of, a sintered material of theinvention) in a refractory lining of an incineration furnace to transferheat by conduction.

Moreover, it was also noticed by the inventors that a second firing inan oxidizing atmosphere, for example in air, of a sintered material ofthe invention, after the first firing in a nitrogen atmosphere(corresponding to the previously described step f)), leads to anexcellent resistance against corrosion. In particular, the dimensionalstability in an oxidizing atmosphere saturated with moisture is veryhigh. Such a property was not predictable.

The second firing may be done at a temperature between 1,000° C. and1,700° C. It would usually last between 1 and 10 hours, depending on thedimensions of the sintered product.

The second firing under oxidizing atmosphere, in particular air, willpassivate the support. It may be performed during the cooling and/or inthe same furnace after firing of said support in a reducing atmosphereof nitrogen at a temperature of 1100° C. to 1700° C.

Fabrication Method

The invention also concerns a method of fabricating a sintered materialbased on silicone carbide (SiC), with a silicon nitride binder (Si₃N₄),said method comprising the following steps in succession:

-   -   A) preparing a charge comprising a particulate mixture        comprising a silicon carbide granulate and at least one boron        and/or calcium compound, a binder optionally being added to said        particulate mixture;    -   B) forming said charge;    -   C) firing said formed charge in a reducing atmosphere of        nitrogen at a temperature in the range 1,100° C. to 1,700° C.;        said boron and/or calcium compound being added in a quantity        which is determined so that the sintered material obtained at        the end of step C) includes a total amount of calcium and boron        in the range 0.05% to 1.5%.

The composition of the starting charge may be determined so as toobtain, at the end of step C), a sintered material which has one orseveral of the optional characteristics of a sintered material accordingto the invention.

At step B), any technique known to form a charge to fabricate a ceramicsmay be used. In particular, step B) may comprise the followingoperations:

-   i) forming the charge by pressing or by casting or slip casting in a    porous or non porous mold, to obtain a preform;-   ii) unmolding said preform;-   iii) drying.

The forming operation may comprise, for instance, an additionalcompacting technique by vibration or tamping

In an embodiment of the invention, the forming operation at step i) maycomprise extrusion.

In an embodiment of the invention, the fabrication method according tothe invention comprises an additional step, following step C), in whichthe sintered material obtained at the end of step C) is fired in anoxidizing atmosphere, preferably at a temperature between 1,000° C. and1,700° C., and possibly for a time in the range between 1 and 10 hours,depending on the dimensions of the mass of sintered material.

The second firing step may be done as a post-treatment, after partial orcomplete cooling of the sintered material obtained at the end of thefirst firing step.

It could also be done in the continuity of the first firing step,through the injection of an oxidizing gas, for example air, without anysubstantial cooling between the two firing steps. The fabrication methodaccording to this embodiment is therefore particularly efficient.

The oxidizing atmosphere preferably contains at least 20% by weight ofoxygen

EXAMPLES

The samples of the following examples A to D were shaped by slip castingin a gypsum mould to form plates.

The ceramic slip was prepared from the constituents according to Table2. After removal from the mould, the green plates were dried at up to100° C. and fired at 1,420° C. under nitrogen atmosphere. The greenplates were thus sintered by reaction with nitrogen to form siliconnitride bond.

The samples of the examples E to H are plates, tiles or bricks whichwere prepared by pressing a mix inside a mould at a pressure of about500 kg/cm² in order to obtain green compacts with the desireddimensions. The ceramic mix was prepared from the constituents accordingto the Table 2′.

After removal from the mould, the plates or tiles or bricks of all theexamples except G were dried at up to 100° C. and fired at 1,420° C.under nitrogen atmosphere.

The product G is a brick made of sintered chromium oxide called Zirchrom60® delivered by Saint-Gobain SEFPRO

Bulk density and open porosity were measured in accordance with ISO5017Standard.

The modulus of rupture in bending (MOR) was evaluated according to theISO5014 Standard.

The steam oxidation test 5 was performed in accordance with ASTM C863 at1000° C.

The corrosion test 6 for reproducing the corrosion in heat recuperativefurnaces for fibre glass making was obtained in the following manner.Three cylindrical samples of 22 mm diameter and 100 mm length were cutfrom the core of each brick, tile or plate to be tested. The samples,placed in a furnace crucible, were immersed in a slag of rasorite (whichis a kind of sodium boride) at a temperature of 1000° C. for 48 hoursunder air atmosphere. The samples were rotated at a speed of 6 rpm. Thevolume was measured before and after corrosion. The corrosion indicatoris equal to the reverse of the average volume loss of the three samplesof the tested brick, tile or plate. The volume loss results from thecorrosive attack by the melt and the vapours. Higher is the corrosionindicator, higher is the corrosion resistance. The Zirchrom 60® is thereference product. Its corrosion indicator is therefore 100.

The thermal conductivity was measured according to test 7. First, thethermal diffusivity a (600° C.) was measured by laser flash method at600° C. under argon in accordance with EN821.2 recommendation. Thethermal conductivity lambda (600° C.) was evaluated according to therelation

Lambda(600° C.)=rho*a(600° C.)*Cp(600° C.)

where rho is the bulk density measured in accordance with ISO5017 and Cpis the thermal heat capacity measured by a calorimeter.

Test 8 is a creep test under flexion at high temperature. The piece ofmaterial to be tested is cut in order to obtain a sample of dimensions70*20*4 mm³ extracted from pieces of 100 mm*100 mm*4 mm, previouslyfired or passivated under air at 1400° C., during a soak time of 5 hoursafter a first firing in a reducing atmosphere of nitrogen.

After grinding, the sample is placed in a furnace equipped with a 3point bending system. The sample is placed on two supports spaced by 50mm, as represented in FIG. 3. A force F is applied on a top part ofdimensions 1 cm width and 2 cm long (width of the sample to be tested)placed at the middle length of the sample. A load of 2 MPa (F divided bythe surface of said top part) is applied at room temperature and thetemperature of the furnace is then increased to 1450° C. at a rate of 10K/min. At the temperature of 1450° C., the load is increased to 50 MPaand maintained during 10 hours. A measurement system with a dial gaugeallows measuring the deflection (or the vertical deformation) of thesample at the mid length of the sample. This deflection is measured inmm, at 1450° C. under the load of 50 MPa. The deflection result is anaverage of 10 samples i.e. 10 tests.

The test 9 is similar to the test 5, but is performed on samples whichhave been fired or passivated under air at 1400° C., during a soak timeof 5 hours after a first firing in a reducing atmosphere of nitrogen.

The test results in the tables 3 and 3′ show that the products B and Dare much more stable in dimension after an oxidation according a verylong duration compared to the reference products A and B, respectively.

It was also found that the D product of the invention exhibits a betterabrasion resistance compared to the reference product C.

The product F of the invention exhibits a smaller decrease of itsthermal conductivity compared to the reference product E.

The product H of the invention exhibits a really high corrosionresistance in accordance with the test 6 compared to the referenceproduct G.

The Modulus of Rupture of the products according to the invention isalso noteworthy. Therefore, the invention also concerns the use of arefractory shaped product according to the invention (comprising, ormade of, a sintered material of the invention) to increase the modulusof rupture in bending. The standard for measurement is ISO5014.

TABLE 2 A B D According Product C Product to of the Reference of theUS4990469 invention product invention Mix formulation (pourcentages inweight) SiC grains and powder 0.2 to 1 mm 81 80.2 SiC Grains and powder20 to 150 μm 40 39.6 SiC Grains and powder 0.1 to 10 μm 37.5 37.2Silicon metal powder 0.5 to 50 μm 17 16.9 16 16 Calcined Refractory Clay3 3 Boron carbide powder(B4C >95%) 0.8 0.8 diameter of grains: 95% lessthan 45 μm Iron oxide powder 1 to 75 μm 0.5 0.5 Calcined alumina powder5 5 0.4 to 10 μm Mineral formulation 100 100 100 100 Additives(pourcentages in weight relative to total mineral formulation) Fluidizersuch as water and sodium 0.3 0.3 0.2 0.2 hydroxide Water 12.5 12.5 6 6

TABLE 2′ F H E Product Product Reference of the of the product inventioninvention Mix formulation (pourcentage in weight) SiC grains and powder0.2 to 3 mm 65 64.5 72 SiC Grains and powder 21 20.8 14.5 90% <200 μm %Silicon metal powder 0.5 to 75 μm 14 13.9 13 Boron carbide powder(B4C >95%) 0.8 0.5 diameter of grains 95% lower than 45 μm Iron oxidepowder 1 to 75 μm Calcined alumina powder 0.4 to 10 μm Mineralformulation 100 100 100 Additives (pourcentages in weight relative tototal mineral formulation) Calcium lignosulfonate binder +2.0 +2.0 +2.0Cellulose binder +0.5 +0.5 +0.5 Water +3.0 +3.0 +3.0

TABLE 3 examples A B D According to Product of the C Product of the U.S.Pat. No. 4,990,469 invention Reference product invention Shape of theproduct 50*50*4 mm plates 50*50*4 mm plates 50*50*4 mm plates 50*50*4 mmplates Test 1 Bulk density 2.82 2.83 2.65 2.67 Test 1 Open porosity %12.0 12.1 15.0 15.0 Test 2 Modulus of rupture MPa at 175 180 40 42ambiant Test 3 Si3N4 α 8 5 Si3N4 β 14 17 SiAlON β′ Si2ON2 3 2 Test 4 NLECO analysis 8.5 8.7 8 8.2 SiC LECO analysis 72 71 75 76 O LECOanalysis 1.5 1.6 Test 5 Δ m/m (%) after 500 h +3.0+ +1.9 Δ v/v (%) after500 h +2.5 +0.2 +3.5 +0.5 Test 8 Measurement after firing under 0.35 +/−0.1 0.25 +/− 0.05 air 1400° C./5 h soak: Creep tests under flexion at1450° C./50 MPa load; Deflection after 10 h in mm Test 9 Measurementafter firing under +1.5% +0.7% air 1400° C./5 h soak: Δ m/m (%) after800 h at 1000° C.

TABLE 3′ examples F G = H E Product of Zirchrom 60 Product of Referenceproduct the invention Reference product the invention Shape of theproduct Tile Tile bricks bricks Dim 20*10 0*100 mm Dim 20*10 0*100 mm75*230*230 mm 75*230*230 mm Test 1 Bulk density 2.70 2.70 3.85 2.62 Test1 Open porosity % 14.0 14.0 14.0 16.5 Test 2 Modulus of rupture 55 50MPa at ambiant test 3 Si3N4 α 8 4 1 Si3N4 β 12 18 15 SiAlON β′ Si2ON2 <2<2 Test 4 N LECO analysis 7.9 7.7 8 SiC LECO analysis 79 78.5 78 CR2O3chemical analysis 62 ZrO2 chemical analysis 10 O LECO analysis 0.5 0.50.9 test 5 Δ m/m (%) after 500 h +4.0 +1.8 Δ v/v (%) after 500 h +1.6+0.2 Test 6 corrosion test with 100 400 rasorite at 1000° C./48 hCorrosion index Id Test 7 Thermal conductivity at 20 25 600° C. in w/m ·K by laser flash method Loss in % after steam 8 18 oxidation accordingto test 5 but after 300 h

1-24. (canceled)
 25. A method for manufacturing a refractory shapedproduct comprising: slip casting or pressing in a mold at least amaterial based on silicon carbide (SiC) reactively sintered between1,100° C. and 1,700° C. to form a silicon nitride binder Si₃N₄,including 0.05% to 1.5% of boron, as a percentage by weight, theSi₃N₄SiC weight ratio being in the range 0.05 to 0.45, wherein therefractory shaped product is chosen from the group consisting of kilnfurniture or other firing supports; a block, a tile and a tube of anheat exchanger; a block, a tile and a tube of a heat recuperator; ablock, a tile and a tube of a heat regenerator; a block and a tube of amuffle; a block, a tile and a tube of a domestic incineration furnace; ablock, a tile and a tube of a furnace of a glass making factory; ablock, a tile and a tube of a metallurgical furnace; a tile and a tubeof a protection wall; a block of a skid rail of a metallurgical furnace;a block and a tube of an immersion heater; a heater tube; a thermocoupleprotective tube; and a tube for transporting molten metal.
 26. Themethod according to claim 25, wherein the refractory shaped product ischosen from the group consisting of a plate; a post; a setter; a beam; aroller; a block, a tile or a tube of a furnace of a glass making factoryin a location where it is not in direct contact with molten glass; ablock, a tile or a tube of a holding furnace; a block, a tile or a tubeof a smelting furnace for non ferrous metals; a tile or a tube of aprotection wall; and a block of a skid rail of a furnace for the heattreatment of iron or steel bars or of other metallurgical products. 27.The method according to claim 25, wherein the refractory shaped producthas a thickness less than 1/10 of its length and/or of its width. 28.The method according to claim 27, wherein the refractory shaped producthas a thickness less than 2/100 of its length and/or of its width. 29.The method according to claim 25, wherein the refractory shaped producthas a thickness less than 10 mm.
 30. The method according to claim 25,wherein the percentage by weight of boron in the refractory shapedproduct is 1.2% or less.
 31. The method according to claim 25, whereinthe Si₃N₄/SiC weight ratio in the refractory shaped product is in therange 0.1 to 0.2.
 32. The method according to claim 31, wherein siliconnitride Si₃N₄ in the beta form in the refractory shaped productrepresents, as a percentage by weight, at least 80% of the whole ofsilicon nitride Si₃N₄ in the beta form and in the alpha form.
 33. Themethod according to claim 25, wherein an amount of Si₂ON₂, as apercentage by weight, in the refractory shaped product is less than 5%.34. The method according to claim 25, wherein a percentage by weight ofcalcium in the refractory shaped product is in the range 0.05% to 1.2%.35. The method according to claim 25, wherein the refractory shapedproduct includes at least 0.3% by weight of boron, as a percentage byweight.
 36. The method according to claim 25, further comprisingsubmitting the refractory shaped product to cycling variations oftemperature, each cycle lasting less than 24 hours.
 37. The methodaccording to claim 36, wherein in any cycle, the temperature varies atleast 1000° C. and/or in which any cycle lasts at least 1 hour.
 38. Themethod according to claim 25, further comprising applying the refractoryshaped product in a bottom part of a heat regenerator of a heatrecuperative furnace for making glass.
 39. A method according to claim25, further comprising applying the refractory shaped product in arefractory lining of an incineration furnace to transfer heat byconduction.
 40. The method according to claim 25, further comprisingapplying the refractory shaped product in a refractory lining of anincineration furnace, wherein the refractory shaped product has not beensubmitted to an oxidative post-treatment at a temperature between 1000°C. and 1700° C.
 41. The method according to claim 25, wherein therefractory shaped product is kiln furniture or other firing support onwhich a coating comprising or made of metal hydrate, alumina, aluminasilicate borosilicate, zirconia and mixtures thereof is deposited. 42.The method according to claim 25, wherein said shaped product is kilnfurniture or other firing support free of any coating.
 43. The methodaccording to claim 25, wherein the refractory shaped product is at leastpartly covered with a coating, to avoid peeling of the coating when therefractory shaped product is used as a support to sinter porcelainpieces.
 44. The method according to claim 25, wherein the refractoryshaped product is applied to an object to increase the modulus ofrupture in bending.
 45. The method according to claim 25, wherein theslip casting or pressing in a mold comprise the following steps insuccession: a) preparing a charge comprising a particulate mixturecomprising a silicon carbide granulate and at least one boron compound,a binder optionally being added to said particulate mixture; b) formingsaid charge in a mold; c) compacting said charge in the mold to form apreform; d) unmolding said preform; e) drying said preform, preferablyin air or a moisture-controlled atmosphere; and f) firing said preformin a reducing atmosphere of nitrogen at a temperature in the range 1100°C. to 1700° C.
 46. A firing method in which an item to be fired issupported by a firing support during firing, said firing supportcomprising a refractory product manufactured by the method of claim 27and being free of any coating, at least on the surface in contact withsaid item, and wherein said firing support has not been passivated. 47.A firing method in which an item to be fired is supported by a firingsupport during firing, said firing support comprising a refractoryproduct manufactured by the method of claim 27 and being free of anycoating wherein said firing support has been passivated between 800° C.and 1600° C.
 48. A method according to claim 31, wherein an amount ofSi₂ON₂, as a percentage by weight, in the refractory shaped product isless than 5%.
 49. The method according to claim 31, wherein thepercentage by weight of boron in said the refractory shaped product is1.2% or less.
 50. The method according to claim 31, wherein therefractory shaped product includes at least 0.3% by weight of boron, asa percentage by weight.
 51. The method according to claim 32, wherein anamount of Si₂ON₂, as a percentage by weight, in said the refractoryshaped product is less than 5%.
 52. The method according to claim 32,wherein the percentage by weight of boron in said the refractory shapedproduct is 1.2% or less.
 53. The method according to claim 32, whereinthe refractory shaped product includes at least 0.3% by weight of boron,as a percentage by weight.
 54. The method according to claim 48, whereinthe percentage by weight of boron in said the refractory shaped productis 1.2% or less.